The skirt of a piston unavoidably contacts the inside of a cylinder sleeve of an internal combustion engine. Therefore, improvement in the abrasion resistance and scorching resistance of the outer surface of a piston is sought. Japanese Patent Application Laid-Open Publication No. 2008-56750 (JP 2008-56750 A) has disclosed a remedial measure with this object.
JP 2008-56750 A discloses a multilayer lubrication coating composition comprising an upper-layer coating composition and a lower-layer coating composition, wherein the upper-layer coating composition comprises 50 to 70 wt % of at least one of an epoxy resin and a polyamide-imide resin as a bonding resin; 5 to 20 wt % of boron nitride as a solid lubricant; and 15 to 30 wt % of at least one of silicon nitride and alumina as hard particles; and the lower-layer coating composition comprises 50 to 70 wt % of at least one of an epoxy resin and a polyamide-imide resin as bonding resins; 15 to 30 wt % of polytetrafluoroethylene as a solid lubricant; and 5 to 20 wt % of molybdenum disulfide. This coating composition can improve abrasion resistance and scorching resistance through the action of a solid lubricant, such as molybdenum disulfide.
A multilayer lubrication coating composition of this configuration achieves abrasion resistance and scorching resistance provided that it is sound.
Depending on the type and operating environment of an engine, however, foreign matter, such as sand, may be mixed into a lubricating oil. This is a concern when using a general-purpose engine without a filter with the object of lower cost, or when running a general-purpose engine in a sandstorm.
When foreign matter is mixed into a lubricating oil, this foreign matter becomes a grinding material, which damages the multilayer lubrication coating composition and exposes the aluminum substrate. When this happens, abrasion and scorching progress and cause the problem of shortening the life of the piston. In the case that the contact pressure between a piston and a cylinder sleeve is greater than a permissible pressure, this pressure damages the multilayer lubrication coating composition, exposes the aluminum substrate, and causes a similar problem.
Therefore, there is a need for a technology capable of extending the life of a metal part coated by a multilayer lubrication coating composition even when this multilayer lubrication coating composition is damaged.